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Sudden Drop In Mpg


ejd
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I have a 95 Mack CH613 that had been getting over 6.5 MPG until recently. Over the past few weeks my MPG has decreased to only 4.5 MPG. I have replaced the air filter, the engine is not smoking, and my last oil change was about 5000 miles ago. Any ideas on what could be the issue. I am open to any suggestions

Thanks

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Start with pressure testing the air/air cooler and all hoses up to and including the head with 25 psi and you may find some leaks. If you dont have the equipment then take to some one who has and if they or you cant get the valves to seal in all cylnders then turn engine slow untill you pressure comes up. There will be a place were all cylinders will have intakes or exhausts valves closed. Some times you will find intake gaskets leaking as well as the air compressor hose leaking. Dont let some one tell you it want work.

glenn akers

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did you replace the fuel filters, water in the fuel, bad batch of fuel, pullin different loads or routes? any codes coming up how many miles

Fuel filter has about 5000 miles on them. This engine has 280 horse power. I have been traveling about 160 miles per day pulling 49000 pound loads. Prior to that i was running empties and lighter loads locally. I figure it is the differance in the weight of the loads but just wanted to be sure someone is not stealing fuel from me or that it could be due to engine problems. The truck runs very well with no other problems.

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Start with pressure testing the air/air cooler and all hoses up to and including the head with 25 psi and you may find some leaks. If you dont have the equipment then take to some one who has and if they or you cant get the valves to seal in all cylnders then turn engine slow untill you pressure comes up. There will be a place were all cylinders will have intakes or exhausts valves closed. Some times you will find intake gaskets leaking as well as the air compressor hose leaking. Dont let some one tell you it want work.

There will be a place were all cylinders will have intakes or exhausts valves closed.

Some people would call you nuts for saying that, but you and I know better don't we! :)

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There will be a place were all cylinders will have intakes or exhausts valves closed.

Some people would call you nuts for saying that, but you and I know better don't we! :)

I have did for serval years and its not by the book but it will help find a leak some times you cant see. And another thing a few bubbles of soapy water at the air/air cooler joints is no reason to replace a cooler. glenn

glenn akers

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I have been traveling about 160 miles per day pulling 49000 pound loads. Prior to that i was running empties and lighter loads locally. I figure it is the differance in the weight of the loads but just wanted to be sure someone is not stealing fuel from me or that it could be due to engine problems.

Personally I think your last statement is a distinct possibility. :angry:

Can you use a dipstick and check your levels after a run and before your next one. Do this over a lot of runs.

Most of my mates reckon they are more fuel efficient with a load ON.

You may have a fuel leak, but this would be evident, so check the underside of the bonnet (hood), all around and under the engine.

Rod.

Proud owner of;

1961 Mack B61 prime mover.

1981 International ACCO 1810C DualCab Fire Truck

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Thanks for all of the suggestions. I actually think I have the problem figured out. I put a hub-ometer on the truck last week and it seems that my odometer is off! Using the mileage from the hub-ometer I am right back where I should be.

Thanks!

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  • 2 weeks later...

LocomotiveBreath is onto something. Now is the time of year the fuel gets cut with additives for winter. I have experienced a drop in MPG as well the last couple weeks. But even pulling full loads(80,000lbs. gross) I get at least 5.5 MPG. 4.5 seems very low. If your odometer is off, wasn't it off before when you were clocking 6.5 MPG? If you got new drive tires that could alter your calculations.

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